Power amp ratings

Jetfire

So, if I have a power amp which is say.. 300 watts going into a cab which is rated 8 ohms and 90 watts.. on a scale of 1 - 10, how bad an idea is that?

ESBlonde

Pretty bad, there are lots of variables including what make of speaker (some are more conservatively rated) what cabinet is it in (each cone will have a perfect match in a closed/reflex cab that will aid loading). That type of power amp (valve or transistor). The type of signal you are feeding it and is the signal very clean and without dynamics.

The term RMS refers to a constant level signal over a broad(ish) spectrum which music just isn't, so a clean clear undynamic limited signal can be fed into the cone in pulses (music beats) at several times the RMS power for reasonable sustained periods. BUT a raw guitar signal is none of those things and outside of just pootling about gently at home the risks are that a large peak (which the amp can easily pass in excess of that 300w) which will pop the voice coil out of the gap. Or a distorted signal (where the high voltage/current holds the voice coil still so theres little cooling and dissipates the energy as excess heat) will damage the loudspeaker soon enough.

What are the components (make and model) and what are the intended uses.

Jetfire

Essentially, its a Mesa Boogie 1 x 12 with a C90 in it with a power amp (possible loan of a Matrix one) but not 100% sure of the wattage...

ICBM

I always think this is a very bad idea, but a lot of people seem to use this sort of amp with cabinets which are nowhere near rated high enough and don't blow them - presumably they're just careful with the volume. I'm just pessimistic where musicians being involved and being careful are required to occur at the same time, especially in a gig situation.

If you really crank the amp up you *will* blow the speaker, no question.

Danny1969

It's no different from having a car that will do 140mph but driving down a small road where your likely to crash if you go much faster than 60. Just common sense, if you don't crank the shit out of it your be fine

ICBM

Danny1969 said:

It's no different from having a car that will do 140mph but driving down a small road where your likely to crash if you go much faster than 60. Just common sense, if you don't crank the shit out of it your be fine

The difference is that when you're driving fast you get instant feedback on whether you're going too fast. With a speaker, you don't always - you can easily go over the power rating without it sounding very different, especially with a distorted sound. Also the sensation of speed in a car increases faster than the real speed, which is the opposite of power input to speakers where you need at least double the power to even make much audible difference.

It's definitely fine at low volume - the problem is that at gig volume it will be very difficult to tell the difference by ear between an output of say 75W and 150W… but that will make a huge difference to a 90W speaker.